Dendritic cells (DC) are unique among antigen-presenting cells (APC) by virtue of their potent capacity to activate immunologically naive T cells (Steinman, 1991). DC express constitutively, or after maturation, several molecules that mediate physical interaction with and deliver activation signals to responding T cells. These include class I and class II MHC molecules CDSO (B7-1) and CD86 (B7-2); CD40; CD11a/CD18 (LFA-1); and CD54 (ICAM-1) (Steinman, 1991; Steinman et al., 1995). DC also secrete, upon stimulation, several T cell-stimulatory cytokines, including IL-1-beta, IL-6, IL-8, macrophage-inflammatory protein-1-alpha (MIP-1-alpha), and MIP-1-delta (Matsue et al., 1992; Kitajima et al., 1995; Ariizumi et al., 1995; Caux et al., 1994; Heufler et al., 1992; Schreiber et al., 1992; Enk et al., 1992; Mohamadzadeh et al., 1996). Both of these properties, adhesion molecule expression and cytokine production are shared by other APC (e.g., activated macrophages and B cells), which are substantially less competent in activating naive T cells.
T cell activation is an important step in the protective immunity against pathogenic microorganisms (e.g., viruses, bacteria, and parasites), foreign proteins, and harmful chemicals in the environment. T cells express receptors on their surfaces (i.e., T cell receptors) that recognize antigens presented on the surface of antigen-presenting cells. During a normal immune response, binding of these antigens to the T cell receptor initiates intracellular changes leading to T cell activation. DC express several different adhesion (and co-stimulatory) molecules, which mediate their interaction with T cells. The combinations of receptors (on DC) and counter-receptors (on T cells) that are known to play this role include: a) class I MHC and CD8, b) class II MHC and CD4, c) CD54 (ICAM-1) and CD11a/CD18 (LFA-1), d) ICAM-3 and CD11a/CD18, e) LFA-3 and CD2, f) CD80 (B7-1) and CD28 (and CTLA4), g) CD86 (B7-2) and CD28 (and CTLA4) and h) CD40 and CD40L (Steinman et al., 1995). Importantly, not only does ligation of these molecules promote physical binding between DC and T cells, it also transduces activation signals.
The dendritic cell (DC) orchestrates several critical steps in the development of an adaptive immune response. DCs communicate information regarding the antigenic state of the peripheral tissues to the local lymph nodes. Upon detection of both pathogen-derived and endogenous “danger signals”, the DC physiologically adapts to its microenvironment by undergoing a genetic program known as “maturation” in order to direct an effective T cell response. The unique machinery of the DC allows it, not only to induce the activation of naïve T cells, but also to regulate their subsequent phenotype and function. These impressive attributes make the DC an ideal choice for their exploitation as natural adjuvants in cancer vaccine development. However, the limited successes of recent clinical trials indicate that current DC therapeutic strategies are in need of further refinement if DC immunotherapy is to be included in the cancer treatment arsenal alongside the more traditional modalities of chemo- and radiotherapy. This translation of DC vaccine development into the clinic will rely significantly upon advancements in our understanding of basic DC biology.
One of the critical deficiencies of DC-based vaccines is their transient nature. The activation state and the longevity of DCs are significantly limited. Less than 24 hours following exposure to bacteria-derived lipopolysaccharide (LPS), DCs terminate synthesis of the IL-12 cytokine and become refractory to further stimuli. This implies that the cytotoxic T lymphocyte (CTL) activation potential of DCs is severely compromised a relatively short time following its activation. Vaccine studies indicate that the survival of antigen-pulsed DCs within the draining lymph node is dramatically reduced 48 hours following their delivery and undetectable by 72 hours. These findings justify the need for alternative strategies for DC vaccine design, such as the development of genetically altered DCs that can circumvent physiological regulatory mechanisms and exhibit enhanced immunostimulatory properties for the treatment of cancer and other diseases.